The report shows that children as young as 11 regularly drink alcohol. It found that 13% of 11-year-old boys and 8% of 11-year-old girls drink alcohol every week.

Some 10% of boys and 6% of girls have been drunk at least twice by the age of 11. By the time they reach 15, more than half of teenagers have been drunk.

Rates of smoking are relatively high. Around 2% of youngsters smoke at least once a week by the age of 11. That increases to over 10% of 13-year-olds and over 25% of 15-year-olds. Girls are much more likely to smoke than boys.

The WHO questioned 162,000 children in 35 countries in Europe and North America as part of its health behaviour in school-aged children (HBSC) project.

It confirmed that Britain has some of the highest rates of child obesity in the World.

But it also revealed that many youngsters are very conscious about how they look and how much they weigh.

Some 16% of girls and 10% of boys in Wales have gone on a diet by the time they are 11. The figures for England and Scotland are only slightly lower.

Most young cannabis users

1. Greenland

2. Switzerland

3. Canada

4. England

5. Scotland

8. Wales

Ranking based on survey of 15-year-olds

The report suggests young Britons are less likely to eat healthily than teenagers in many other countries.

Less than a third of teenagers eat fruit or vegetables every day.

Children in Scotland are more likely to eat sweets and consume fizzy drinks every day compared to children living in almost every other country surveyed.

The report also shows that British teenagers spend hours in front of televisions and computers each week.

For instance, more than a quarter spend over four hours watching television each day during the week. That rises to close to half at weekends.

But researchers insisted that the report did have some positive findings.

"It is not all bad news," said Dr Candace Currie, one of the researchers.

"Rates of smoking have fallen dramatically especially among boys and physical activity rates are comparatively high. There is some really good work going on."